This invention relates to a retrofocus projection lens that projects a magnified image of an image displayed on an image source onto a screen, and more particularly, to a compact and lightweight retrofocus projection lens that may be used as a liquid crystal projector to project a magnified image of an image displayed on a liquid crystal panel onto a screen.
FIG. 22 is a schematic diagram of the optical system of a conventional liquid crystal projector. In this liquid crystal projector, a light beam 20 that has emerged from a lamp 1 and been reflected by an elliptical mirror 2, is reflected by a reflecting mirror 3, and converted to a parallel beam 21 by passing through a collimator lens 4. This parallel beam 21 is split into beams of different colors 21R, 21G, 21B, which respectively pass through condensing lenses 6R, 6G, 6B so as to impinge on light valves 7R, 7G, 7B consisting of liquid crystal panels. The light beams modulated by the images formed by the liquid crystal panels are then recombined into one light beam 22 by dichroic mirrors 8G and 8GB, converted into a projecting beam 23 by a projection lens 9, and projected as a magnified color image on a screen 10. In the FIG. 22, 11 and 12 are reflecting mirrors.
A projection lens of this type for use with a liquid crystal projector is disclosed for example in Japanese Patent Kokai Publications No. 173066/1993 and No. 188287/1993. The projection lens used for the liquid crystal projector must have a long back focal length so that the color synthesizing dichroic mirrors can be inserted. It is also required that it has a high image resolving power, and that it improves distortion and magnification color aberration. It is further required that the number of the lens elements is small, the overall length of the projection lens is small and the lens diameter is small.
However, the projection lenses disclosed in these publications consist of 8 or 12 lens elements. Consequently, due to the large number of lens elements, the overall length of the projection lens was too long, and as the lens diameter also increases, it was difficult to make the projection lens small and lightweight.
A retrofocus lens comprising only 6 lens elements and having a long back focal length compared to focal length is disclosed in Japanese Patent Kokai Publication No. 49623/1974. However, this retrofocus lens is a photographic lens, and its performance characteristics such as field angle, F number and back focal length were not suitable for a liquid crystal projector.